The invention relates to an apparatus for the drying section of a paper machine.
A prior art apparatus has been used in the drying section of a paper or paperboard making machine, in which apparatus the web follows the drying fabric, preferably a felt, in such a way that on the bottom cylinders of a drying cylinder group the web runs as the outermost element, whereas on the top cylinders the web runs between a drying cylinder and a drying fabric. There thus exists a risk especially on the bottom cylinders of the loosening of the paper web, particularly at high web speeds. The web also tends to loosen from the surface of the drying fabric in the area of free transfer which occurs in the felt transfer between the top drying cylinder and the bottom roll.
The drying cylinder and the drying fabric induce an air flow that tends to loosen the web from the drying fabric. This loosening problem occurs especially at high web speeds near the top cylinders in a gap between the felt and the cylinder created by the free transfer of the web and felt between a drying cylinder and a roll at a different height. The longer the free transfer is, the more serious are instability problems and the tendency of the web to loosen from the felt surface.
When apparatuses for producing an underpressure zone over the felt and the web are used, the above-mentioned loosening problem can be eliminated. However, the apparatuses for producing the low air pressure zone require a lot of space, and the space problems in turn result in the cylinder distances having to be made relatively long for making the placement of massive suction boxes or beam constructions creating the underpressure zone possible in a space limited by the felt loop between the top cylinders and the bottom cylinders.
The aforementioned prior art apparatuses are described e.g. in U.S. Pat No. 4,809,445. This patent illustrates a suction box apparatus in which the suction box is fitted on the incoming side of the web in the felt transfer between the top cylinder and the bottom roll. The ascending web and the felt transfer from the bottom roll back to the second upper drying cylinder are, in contrast, not provided with a suction box and the loosening problems of the web are more pronounced during this long free transfer area.